1. Field of the Technology
The present invention relates generally to mobile communication devices operating in wireless communication networks, and more particularly to improve handover methods and apparatus for mobile stations.
2. Description of the Related Art
A mobile communication device, such as a mobile station operating in a wireless communication network, may provide for both voice telephony and packet data communications. A mobile station may, for example, be compatible with 3rd Generation (3G) communication standards and utilize Global System for Mobile Communications (GSM), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) wireless network technologies.
All of these communication standards utilize radio frequency (RF) signal detection techniques. In a mobile RF transceiver or mobile station (MS), a handoff or handover procedure exists in order to accomplish continuous communications as the MS is en route between various cell sites of one or more wireless communication networks. The handover procedure utilizes receiver signal strength indicator (RSSI) for serving cell and neighbor cell measurements, and received signal quality (RXQUAL) for serving cell measurements taken within the MS. The MS periodically monitors the RSSI on its active channel and that of its neighboring channels, and sends the selected RSSI information and RXQUAL to a base station (BS) that is part of the wireless communication network, such as a Global System for Mobile Communications (GSM) network. A BS receives RSSI information reported from the MS and decodes and delivers those messages to a base station controller (BSC). The BSC compiles the RSSI and RXQUAL information received from the MS and decides when and if a handover procedure should be initiated for the MS. If the RSSI level and/or RXQUAL on the active channel drops below a predefined threshold level, indicating poor signal-to-noise ratio in the MS receiver, then the BSC will initiate a handover procedure to begin between the active BS and the MS.
In GSM networks, handover procedures are coordinated by the BSC and through communications between the BS and MS by the sending of handover command messages over a fast associated control channel (FACCH). After the BS sends an initial handover command to the MS to begin the handover procedure, a series of handover command messages are exchanged between the MS and the BS (also referred to as a handshaking process). The handover procedure eventually concludes with the MS being assigned to a neighboring channel and/or base station. Each handover command message requires eight continuous FACCH bursts on a full rate traffic channel (TCH) or six continuous FACCH bursts on a half rate TCH. Unfortunately, FACCH bursts are susceptible to instantaneous interference products such as radio interference or radio propagation fading and blocking, and may be missed or corrupted during their short transmission time (approximately 40 ms). When handover commands are lost or damaged, the handover procedure may be prolonged and delayed due to repeat handover attempts, which causes degraded signal quality during the call. In more extreme situations, the communication link may be lost altogether, which results in a dropped call.
Further, much information exchange is required after each and every handover procedure in order to instruct the MS of a new neighboring channel list. In GSM networks, this list is referred to as a broadcast control channel (BCCH) allocation (BA) list. Traditional procedures provide for the transfer of the new BA list following the final handover command message. This further delays the completion of the procedure and delays use of the MS for voice and data communications, and delays valid radio measurements. In GSM networks, the BA list transfer is performed on the slow associated control channel (SACCH) and can take an additional 1.44 s. The additional time required to update the MS with the new BA list can be long enough to cause a drop in the communications link during weak RF signal conditions or in congested RF traffic areas.
Accordingly, what are needed are methods and apparatus for performing a successful handoff between cells so as to overcome the deficiencies in the prior art.